The most recent version of OCaml is 4.06.0.
It was released on 2017-11-03.

What's new?

Some of the highlights in release 4.06 are:

Strings (type string) are now immutable by default.
In-place modification must use the type bytes of byte
sequences, which is distinct from string. This corresponds
to the -safe-string compile-time option, which was introduced
in OCaml 4.02 in 2014, and which is now the default.

Object types can now extend a previously-defined object type, as in
< t; a: int >.

Destructive substitution over module signatures can now express more substitutions, such as
S with type M.t := type-expr
and
S with module M.N := path.

License

Ocaml is Free Software, copyright INRIA, licensed under the
LGPL version 2.1
(with a special exception on static linking).
See the full licence. Members of the
Caml Consortium benefit from a more liberal
license (BSD-like).